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Finding of Prehnite-Pumpellyite Facies Metabasites from the Kurosegawa Belt in Yatsushiro Area, Kyushu, Japan

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Finding of Prehnite-Pumpellyite Facies Metabasites from the Kurosegawa Belt in Yatsushiro Area, Kyushu, Japan Journal ofPrehnite Mineralogical-pumpellyite and Petrological facies metabasites Sciences, in Yatsushiro Volume 107, area, page Kyushu 99─ 104, 2012 99 LETTER Finding of prehnite-pumpellyite facies metabasites from the Kurosegawa belt in Yatsushiro area, Kyushu, Japan * * ** Kenichiro KAMIMURA , Takao HIRAJIMA and Yoshiyuki FUJIMOTO *Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Kyoto 606-8502, Japan ** Nittetsu-Kogyo Co., Ltd, 2-3-2 Marunouchi, Chiyoda, Tokyo 100-8377, Japan. Common occurrence of prehnite and pumpellyite is newly identified from metabasites of Tobiishi sub-unit in the Kurosegawa belt, Yatsushiro area, Kyushu, where Ueta (1961) had mapped as a greenschist facies area. Prehnite and pumpellyite are closely associated with chlorite, calcite and quartz, and they mainly occur in white colored veins or in amygdules in metabasites of the relevant area, but actinolite and epidote are rare in them. Pumpellyite is characterized by iron-rich composition (7.2-20.0 wt% as total iron as FeO) and its range almost overlaps with those in prehnite-pumpellyite facies metabasites of Ishizuka (1991). These facts suggest that the metabasites of the Tobiishi sub-unit suffered the prehnite-pumpellyite facies metamorphism, instead of the greenschist facies. Keywords: Prehnite-pumpellyite facies, Lawsonite-blueschist facies, Kurosegawa belt INTRODUCTION 1961; Kato et al., 1984; Maruyama et al., 1984; Tsujimori and Itaya, 1999; Tomiyoshi and Takasu, 2009). Until now, The subduction zone has an essential role for the global there is neither clear geological nor petrological evidence circulation of solid, fluid and volatile materials between suggesting what type of metamorphic rocks occupied the the surface and inside of the Earth at present. Recent lower grade part of the LBS in above mentioned areas, progress of thermal modeling has helped to interpret the except for the Kamuikotan belt (Ishizuka et al., 1983). In thermal structures of the descending plate in various types other words, this fact means that the thermal structure in of subduction zone (e.g., Peacock and Wang, 1999; Gerya the shallower level of the cold subduction system predict- et al., 2002; Miyazaki and Okumura, 2002). Linking these ed by the thermal modeling was not certified in nature. results with the metamorphic phase petrology can make it Our recent petrological study in the Kurosegawa belt possible to predict what type of metamorphism is going in Yatsushiro area, Kyushu, have newly identified metaba- on in the present day subduction system, e.g., the lawson- sites that suffered the PrP facies metamorphism to the east ite-blueschist (LBS) can be formed from mid-ocean ridge of the LBS area, which was originally reported by Ueta basalt (MORB) at around 30 km depths in the cold sub- (1961). This paper concisely describes the areal extent of duction environment and pumpellyite-actinolite (PA), PrP zone, and the mode of occurrence and chemical com- prehnite-pumpellyite (PrP) or prehnite-actinolite (PrA) positions of prehnite and associated metamorphic miner- facies metabasites can be formed in shallower parts than als. that of the LBS formation (e.g., Peacock and Wang, 1999). GEOLOGICAL SETTING In Japanese Islands, the LBS is mainly reported from the lower grade part of the Kamuikotan belt (e.g., Shiba- The Kurosegawa and Chichibu belts lie between the Usu- kusa, 1989) and in tectonic blocks or small coherent units ki-Yatsushiro Tectonic Line, a western extension of the from the Kurosegawa belt and Renge belt (e.g., Ueta, Median Tectonic Line, and the Butsuzo Tectonic Line in doi:10.2465/jmps.111020c central Kyushu (Fig. 1). Ueta (1961) divided the Tobiishi K. Kamimura, [email protected] Corresponding and Shimotake Formations of Kanmera (1952) in the author Yatsushiro area into two metamorphic zones based on the 100 K. Kamimura, T. Hirajima and Y. Fujimoto Figure 1. Geological sketch map and metamorphic zonation in the Kurosegawa belt in Yatsushiro area, Kyushu. Geological unit boundaries mainly follow Saito et al. (2005). Table 1. Correlation table for geological unit names and metamorphic zonation in the study area LBS, lawsonite-blueschist facies; GS, greenschist facies; PrP, prehnite-pumpellyite facies. mode of occurrence of lawsonite (Table 1); zone 1, char- rocks of Otao unit belonging to Jurassic sedimentary acterized by the occurrence of LBS, is mostly exposed in complex of Saito et al. (2005). We tentatively propose the western half of the Tobiishi Formation, and zone 2, new unit names based on the areal extent of occurrence of characterized by the occurrence of greenstones without metabasites, as Hakoishi and Tobiishi sub-units, from the lawsonite, is mostly in the Shimotake Formation and the viewpoint of metamorphic petrology as described below eastern half of Tobiishi Formation. He proposed that the (Fig. 1 and Table 1). metamorphic grade had increased from the zone 1 to zone The areal extent of Hakoishi sub-unit is mostly cor- 2 and that these metamorphic rocks are the western exten- responding with those of the western half of Tobiishi For- sion of the Sanbagawa belt. mation of Kanmera (1952) and the Hakoishi serpentinite Although any modern geological and petrological mélange unit of Saito et al. (2005). Our geologic field study has not been carried out in this area until the end of mapping revealed that this area is mainly composed of in- th 20 century after Ueta (1961), recently Saito et al. (2005) tercalations of LBS and meta-siliceous rocks, in a 2 × 10 published a detail geologic map. They proposed that ser- km2 extent, and that serpentinite is widely distributed at pentinite mélange with metamorphic and plutonic rocks, the southern edge of the sub-unit, but, to the best of our Paleozoic-Mesozoic strata, and Permian and Jurassic sed- knowledge, it is scarce inside the sub-unit (Ibuki et al., imentary complexes belong to the Kurosegawa belt (Fig. 2008). 1 and Table 1). The areal extent of the Tobiishi sub-unit in this study In this area, metabasites are mainly exposed in the is mostly corresponding with those of the eastern half of Tobiishi Formation of Kanmera (1952), or Hakoishi ser- Tobiishi Formation of Kanmera (1952) and the Otao unit pentinite mélange unit and basaltic lava and volcaniclastic of Saito et al. (2005). The Tobiishi sub-unit is mainly Prehnite-pumpellyite facies metabasites in Yatsushiro area, Kyushu 101 Figure 2. (a) Outcrop photo of close-packed pillow lava (OD249) in the Tobiishi sub-unit. (b) Photomicrograph of metamorphic minerals (Pmp, Chl and Qtz) filling amygdule in a metabasite (OD111), open nicol. (c) Photomicrograph of vein fillings Prh, Ep and Pmp developed in a me- tabasite (OD250), crossed nicols. (d) Backscattered image of prehnite domain of a metabasite (OD250). composed of close-packed pillow lava (Fig. 2a), pillow yama (Fig. 1), i.e., Tobiishi sub-unit. As the mineral as- breccia and hyaloclastite along with subordinate gabbroic semblage of the LBS and its areal extent are mostly iden- and granitic rocks. Most of them still retain igneous tex- tical with zone 1 of Ueta (1961), the subsequent description tures and minerals in their matrix, such as ophitic texture will concentrate on the mode of occurrence of prehnite and cavities, and they almost lack distinctive penetrative and associated minerals in the Tobiishi sub-unit, where foliation. Metamorphic minerals, such as prehnite and Ueta (1961) defined as zone 2 suffered the greenschist fa- pumpellyite, are mainly developed in veins or cavities, cies metamorphism. which are ubiquitously developed in metabasites (Figs. The mineral identification and their chemical analy- 2b-2d). However, neither lawsonite-sodic amphibole nor sis were carried out using a Hitachi S3500H scanning epidote-actinolite- chlorite assemblages were found from electron-probe microanalyzer (EPMA) equipped with an metabasites. Therefore, we tentatively call the eastern half EDAX energy dispersive X-ray analytical system at Kyo- of the Tobiishi Formation of Kanmera (1952) as Tobiishi to University. The accelerating voltage and beam current sub-unit (Fig. 1). were maintained at 20 kV and 500 pA, respectively. In metabasites of the Tobiishi sub-unit, prehnite, PETROGRAPHY AND MINERALOGY pumpellyite, chlorite, celadonite, albite, quartz and calcite are main metamorphic minerals along with minor epidote, Petrological study based on more than 200 thin section actinolite, adularia, and titanite in veins and amygdules observation revealed that the LBS and associated rocks (Figs. 2b-2d). are distributed to the west of the Mt. Yayama, i.e., Hakoi- Most veins, mainly less than 1 mm and up to a few shi sub-unit, and prehnite is a predominant metamorphic cm in width, show white color and rare ones show green- mineral in metabasites distributed to the east of Mt. Ya­ ish-yellow color in hand specimens. White veins are 102 K. Kamimura, T. Hirajima and Y. Fujimoto Table 2. EPMA data of representative metamorphic minerals in metabasites of Tobiishi and Hakoishi sub-units All iron was calculated as Fe2O3 for prehnite and as FeO for pumpellyite. Ferric/ferrous ratio of iron for pumpellyite was estimated as Fe3+ + Al = 5.00 for Oxygen = 24.5 p.f.u. mainly composed of calcite, albite, quartz and prehnite, showing greenish pleochroism is richer in iron (~ 7–20 and greenish-yellow veins are mainly composed of wt% as total iron as FeO) than those (~ 7–11 wt% of FeO) pumpellyite, calcite, celadonite and chlorite. Amygdules of yellowish type. are mainly occupied by pumpellyite, chlorite, celadonite, Chlorite also occurs as aggregates of polysynthetic calcite and quartz, but scarce in prehnite and actinolite. twining. It can be distinguished from pumpellyite by its Prehnite shows the yellowish retardation of the first lower refractive index and lower retardation of first order order and the straight extinction (Fig. 2c), and mainly oc- bright - dark gray colors. Furthermore, pleochroism of curs in white colored veins along with quartz, pumpellyite chlorite, such as pale yellow, pale green to colorless, is and minor epidote and calcite.
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